LIG_CaMK_CASK_1

The peripheral plasma membrane protein CASK (Calcium/calmodulin-dependent serine protein kinase CASK; Lin-2 homolog) is a scaffold protein that is involved in synapse formation and signal transduction. It belongs to the membrane-associated guanylate kinase (MAGUK) protein family and was found to be an essential protein in vertebrates. CASK is a multidomain protein comprising several protein interaction domains, including a PDZ, SH3 and calmodulin-dependent protein kinase (CaMK) domain, which target CASK to its many binding partners and mediate complex assembly. The CaMK domain recognizes target proteins such as Caskin-1 and Mint-1 through their CaMK-binding motif, which consists of a hydrophobic core and is situated within the largely unstructured CASK interaction domain (CID).

The CASK-CaMK-binding motif mediates specific binding to the CaMK domain of the CASK protein. The motif is located in an unstructured region of CaMK-binding proteins and binds with an affinity in the low micromolar range (7.5 µM for the motif of Caskin-1) (Stafford,2011). The motif contains an invariant Trp residue that is predicted to insert in a hydrophobic pocket on the surface of the CASK CaMK domain containing Val117. Furthermore, a Val or Ile residue adjacent to the Trp residue is thought to complete the hydrophobic core of the motif. In addition, mutation and deletion studies showed that loss of the N-terminal acidic residue or the invariant C-terminal basic Arg residue impairs binding of the motif, indicating an involvement of these residues in the binding process (Stafford,2011). Several Invertebrates, mainly insects, contain an alternative SP site instead of the conserved acidic residue at the N-terminus of the motif. This might represent a modification site for proline-directed kinases that would mimic an acidic residue after phosphorylation, which would allow regulated induction of motif binding. Binding of the motif might also be regulated by modification of the CaMK domain. Recent studies on the CaMK domain of CASK refute the assumption that it is a catalytically inactive pseudokinase. It was shown that the CaMK domain of CASK is capable of auto-phosphorylation at Ser151 and Ser155. As these residues lie spatially close to the predicted interaction site of the CaMK-binding motif, these modifications might affect motif binding (Mukherjee,2008; Stafford,2011).

The peripheral plasma membrane protein CASK/LIN2 (Calcium/calmodulin-dependent serine protein kinase, CSKP_HUMAN) is a scaffold protein that belongs to the membrane-associated guanylate kinase (MAGUK) protein family and is conserved in vertebrates and invertebrates. In vertebrates, CASK is an essential protein as gene knock out leads to perinatal death in mice (Atasoy,2007), while in invertebrates the gene seems to be dispensable (LaConte,2013). CASK is expressed in several tissues and shows a high expression in neuronal tissue, where it is mainly localized to the pre- and postsynaptic membranes. In this context, CASK was found to regulate both pre- and postsynaptic processes including the formation of presynaptic termini, neurotransmitter release, the maintenance of the morphology of the dendritic spines, and the regulation and localization of ion channels. In addition, CASK contributes to the establishment of cell polarity and can migrate to the nucleus, where it is also involved in the regulation of gene expression, for instance via binding of the T-box transcription factor Tbr-1 (Hsueh,2000; Hsueh,2006).CASK is a multidomain protein that contains a PDZ domain (PF00595), an SH3 domain (PF07653) and a guanylate kinase (GK) domain (PF00625). The presence of two additional L27 domains (PF02828) is characteristic for the p55 subfamily (Hsueh,2006). In addition, CASK contains a unique N-terminal calmodulin-dependent protein kinase (CaMK) domain (PF00069, 3MFR, Mukherjee,2010). Because it lacks the Mg2+ binding motif, which is considered to be required for the catalytic activity of protein kinases, the CaMK domain of CASK was regarded as a non-active pseudokinase involved only in non-catalytic protein-protein interactions. However, recent studies refuted this by showing CaMK auto-phosphorylation and phosphorylation of the membrane adhesion protein Neurexin-1 (NRX1A_HUMAN) by CASK, indicating a further involvement of this protein in phosphorylation processes (Mukherjee,2008). These different interaction domains, for which different binding partners have been identified, allow CASK to bind multiple targets simultaneously and serve as a scaffold protein that mediates the assembly of protein complexes.A validated binding partner of the CASK CaMK domain is Mint-1/APBA1 (Neuronal Munc18-1-interacting protein 1, APBA1_HUMAN), which, together with CASK and Veli (MALS/Lin-7) proteins, forms tripartite complexes that bind to the cytoplasmic tail of Neurexin-1 at pre- and postsynaptic sites (Hata,1996). The CASK-Veli-Mint-1 complex is involved in regulation of synaptic signal transduction. At the presynapse it forms a quadripartite complex with liprin-alpha that plays a role in vesicular release (Olsen,2005), while at the postsynapse it directs the transport of NMDA receptors along microtubules (Setou,2000).Regulation of CASK-Veli-Mint-1 complex assembly involves Caskin-1 (CASK-interacting protein 1, CSKI1_HUMAN). This brain-specific multidomain protein Caskin-1 competes with Mint-1 for binding to the CaMK domain of CASK and forms distinct tripartite complexes with CASK and Veli proteins to modulate the function of CASK (Tabuchi,2002). Competitive binding of Caskin-1 and Mint-1 to CASK depends on a common motif in the two competitors that binds to a hydrophobic pocket on the CaMK domain of CASK. The CaMK-binding motif is localized within the CASK interaction domain (CID) of the ligand proteins, an unstructured region that was initially identified as a CaMK-binding domain in Mint-1 (Borg,1999). More recently, the TIAM-1 protein (T-lymphoma invasion and metastasis-inducing protein 1, TIAM1_HUMAN) was found to contain a similar motif sequence and interact with CASK, which might be relevant considering its high expression level in brain tissue and overlapping functions with CASK, including dendritic spine development (Ehler,1997, Tolias,2007).